Magnetic disk device and method

ABSTRACT

According to one embodiment, in a magnetic disk, a plurality of first servo sectors is arranged at intervals in the circumferential direction. Each of the plurality of first servo sectors includes a first area and a second area. First information including a preamble, a servo mark, and a Gray code is written in the first area. The second area is disposed after the first area in a write and read direction along the circumferential direction, and second information including a burst pattern is written in the second area. The plurality of first servo sectors includes a plurality of second servo sectors and a plurality of third servo sectors. The circumferential length of a first area included in each of the plurality of third servo sectors is longer than the circumferential length of a first area included in each of the plurality of second servo sectors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2022-126097, filed on Aug. 8, 2022; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a magnetic disk deviceand a method.

BACKGROUND

In a general magnetic disk device, a magnetic disk includes a pluralityof servo areas in which servo information is written at intervals in acircumferential direction of the magnetic disk. Then, in thecircumferential direction of the magnetic disk, an area between servoareas is set as a data area where data can be written.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an example of a configurationof a magnetic disk device according to a first embodiment;

FIG. 2 is a schematic diagram illustrating an example of a configurationof a magnetic disk according to the first embodiment;

FIG. 3 is a diagram for describing an example of a positionalrelationship between a read head and a write head according to the firstembodiment;

FIG. 4 is a diagram illustrating an example of a configuration of anormal servo sector according to the first embodiment;

FIG. 5 is a diagram illustrating an example of a configuration of ashort servo sector according to the first embodiment;

FIG. 6 is a schematic diagram for describing a Gray code written in aGray code area of a normal servo sector according to the firstembodiment;

FIG. 7 is a schematic diagram for describing Gray codes written in Graycode areas of short servo sectors according to the first embodiment;

FIG. 8 is a schematic table for describing a relationship among the typeof a servo sector, the type of an operation being executed, and apattern of a servo gate to be used according to the first embodiment;

FIG. 9 is a schematic diagram for describing an example of a waveform ofa servo gate of a pattern NormalSG1 used when a magnetic head passesthrough a normal servo sector in the write operation of the firstembodiment;

FIG. 10 is a schematic diagram for describing an example of a waveformof a servo gate of a pattern ShortSG used when the magnetic head passesthrough a short servo sector in the write operation of the firstembodiment;

FIG. 11 is a schematic diagram for describing an example of a waveformof a servo gate of a pattern NormalSG1 used when the magnetic headpasses through a normal servo sector in the read operation of the firstembodiment;

FIG. 12 is a schematic diagram for describing an example of a waveformof a servo gate of a pattern NormalSG2 used when the magnetic headpasses through a short servo sector in the read operation of the firstembodiment;

FIG. 13 is a schematic diagram for explaining an example of the waveformof a servo gate of a pattern SeekSG1 used when the magnetic head passesthrough a normal servo sector in the seek operation of the firstembodiment;

FIG. 14 is a schematic diagram for explaining an example of the waveformof a servo gate of a pattern SeekSG2 used when the magnetic head passesthrough a short servo sector in the seek operation of the firstembodiment;

FIG. 15 is a flowchart illustrating an example of the operation ofdetermining a demodulation method of servo information of each servosector by a controller according to the first embodiment;

FIG. 16 is a schematic diagram illustrating an example of aconfiguration of a magnetic disk according to a second embodiment;

FIG. 17 is a diagram illustrating an example of a configuration of anormal servo sector according to the second embodiment;

FIG. 18 is a schematic diagram for describing a Gray code written inGray code areas according to the second embodiment;

FIG. 19 is a schematic table for describing a relationship between thetype of an operation being executed and a pattern of a servo gate to beused according to the second embodiment;

FIG. 20 is a schematic diagram for describing an example of a waveformof a servo gate of a NormalSG3 pattern used when a magnetic head passesthrough a normal servo sector in the write operation of the secondembodiment;

FIG. 21 is a schematic diagram for describing an example of a waveformof a servo gate of a pattern NormalSG3 used when the magnetic headpasses through a normal servo sector in the read operation of the secondembodiment;

FIG. 22 is a schematic diagram for explaining an example of the waveformof a servo gate of a pattern SeekSG3 used when the magnetic head passesthrough a normal servo sector in the seek operation of the secondembodiment; and

FIG. 23 is a flowchart illustrating an example of the operation ofdetermining a demodulation method of servo information of each servosector by a controller according to the second embodiment.

DETAILED DESCRIPTION

According to the present embodiment, a magnetic disk device includes amagnetic disk, a magnetic head, and a controller. A plurality of firstservo sectors is arranged in the magnetic disk at intervals in thecircumferential direction. The magnetic disk includes a plurality ofdata areas provided between two first servo sectors adjacent to eachother in the circumferential direction. Each of the plurality of firstservo sectors includes a first area and a second area. First informationincluding a preamble, a servo mark, and a Gray code is written in thefirst area. The second area is disposed after the first area in a writeand read direction along the circumferential direction, and secondinformation including a burst pattern is written in the second area. Theplurality of first servo sectors includes a plurality of second servosectors and a plurality of third servo sectors. One or more third servosectors of the plurality of third servo sectors are arranged between twoadjacent second servo sectors of the plurality of second servo sectors.The circumferential length of a first area included in each of theplurality of third servo sectors is longer than the circumferentiallength of a first area included in each of the plurality of second servosectors. The controller demodulates the first information and the secondinformation when the magnetic head passes through one of the pluralityof second servo sectors in write operation and demodulates the secondinformation without demodulating the first information when the magnetichead passes through one of the plurality of third servo sectors. Thewrite operation is an operation of writing data in one or more of theplurality of data areas using the magnetic head. The controllerdemodulates the first information and the second information when themagnetic head passes through one of the plurality of second servosectors in read operation and demodulates the first information and thesecond information when the magnetic head passes through one of theplurality of third servo sectors. The read operation is an operation ofreading data from one or more of the plurality of data areas using themagnetic head.

Hereinafter, a magnetic disk device and a method according toembodiments will be described in detail with reference to theaccompanying drawings. Note that the present invention is not limited bythese embodiments.

First Embodiment

FIG. 1 is a schematic diagram illustrating an example of a configurationof a magnetic disk device 1 according to a first embodiment.

The magnetic disk device 1 is connected to a host 2. The magnetic diskdevice 1 can receive an access command such as a write command or a readcommand from the host 2.

The magnetic disk device 1 includes a magnetic disk 11 having a magneticlayer formed on a surface thereof. The magnetic disk device 1 writesdata to the magnetic disk 11 or reads data from the magnetic disk 11 inresponse to an access command.

Data is written and read via a magnetic head 22. Specifically, inaddition to the magnetic disk 11, the magnetic disk device 1 includes aspindle motor 12, a ramp 13, an actuator arm 15, a voice coil motor(VCM) 16, a motor driver integrated circuit (IC) 21, the magnetic head22, a hard disk controller (HDC) 23, a head IC 24, a read and writechannel (RWC) 25, a processor 26, a RAM 27, a flash read only memory(FROM) 28, a buffer memory 29, and a counter 31.

The magnetic disk 11 is rotated at a predetermined rotation speed by aspindle motor 12 attached coaxially. The spindle motor 12 is driven bythe motor driver IC 21.

The processor 26 controls the rotation of the spindle motor 12 and therotation of the VCM 16 via the motor driver IC 21.

The magnetic head 22 writes and reads information to and from themagnetic disk 11 using a write head 22 w and a read head 22 r includedtherein. The magnetic head 22 is attached to a distal end of theactuator arm 15. The magnetic head 22 is moved in the radial directionof the magnetic disk 11 by the VCM 16 driven by the motor driver IC 21.Note that, as for the write head 22 w and the read head 22 r included inthe magnetic head 22, a single magnetic head 22 may include a pluralityof write heads 22 w and/or read heads 22 r.

For example, in cases like when the rotation of the magnetic disk 11 isstopped, the magnetic head 22 is moved to the ramp 13. The ramp 13 holdsthe magnetic head 22 at a position spaced apart from the magnetic disk11.

The head IC 24 amplifies and outputs a signal read from the magneticdisk 11 by the magnetic head 22 during the read operation and suppliesthe read signal to the RWC 25. In addition, the head IC 24 amplifies asignal corresponding to data to be written that is supplied from the RWC25 and supplies the amplified signal to the magnetic head 22 during thewrite operation.

The HDC 23 performs control of transmission and reception of data withthe host 2 via an I/F bus, control of the buffer memory 29, errorcorrection processing of read data, and others.

The buffer memory 29 is used as a buffer for data transmitted to andreceived from the host 2. For example, the buffer memory 29 is used totemporarily store data to be written or data read from the magnetic disk11.

The buffer memory 29 includes, for example, a volatile memory capable ofhigh-speed operation. The type of the memory included in the buffermemory 29 is not limited to a specific type. The buffer memory 29 mayinclude, for example, a dynamic random access memory (DRAM), a staticrandom access memory (SRAM), or a combination thereof. Note that thebuffer memory 29 may include a desired non-volatile memory.

The RWC 25 modulates data to be written that is supplied from the HDC 23and supplies the modulated data to the head IC 24. The RWC 25 alsodemodulates a signal read from the magnetic disk 11 and supplied fromthe head IC 24 and outputs the demodulated signal to the HDC 23 asdigital data.

The processor 26 is, for example, a central processing unit (CPU). TheRAM 27, the flash read only memory (FROM) 28, the buffer memory 29, andthe counter 31 are connected to the processor 26.

The FROM 28 is a nonvolatile memory. The FROM 28 stores firmware(program data), various operation parameters, and others. Note that thefirmware may be stored in the magnetic disk 11.

The RAM 27 includes, for example, a DRAM, an SRAM, or a combinationthereof. The RAM 27 is used as an operation memory by the processor 26.The RAM 27 is used as an area in which the firmware is loaded and anarea in which various types of management data are temporarily stored.

The processor 26 performs overall control of the magnetic disk device 1in accordance with the firmware stored in the FROM 28 or the magneticdisk 11. For example, the processor 26 loads the firmware from the FROM28 or the magnetic disk 11 to the RAM 27 and executes control of themotor driver IC 21, the head IC 24, the RWC 25, the HDC 23, and othersin accordance with the loaded firmware.

The counter 31 is a timer circuit whose count value increases with time.The processor 26 uses the counter 31 in order to determine the timing ofvarious operations. The type of the counter 31 is not limited to aspecific type. In one example, the counter 31 may be avoltage-controlled oscillator (VCO) counter.

Note that a configuration including the HDC 23, the RWC 25, theprocessor 26, and the counter 31 can also be regarded as a controller30. In addition to these components, the controller 30 may include othercomponents (such as the RAM 27, the FROM 28, or the buffer memory 29).The counter 31 may be provided outside the controller 30.

Furthermore, the firmware program may be stored in the magnetic disk 11.Some or all of the functions of the processor 26 may be implemented by ahardware circuit such as a field-programmable gate array (FPGA) or anapplication specific integrated circuit (ASIC).

FIG. 2 is a schematic diagram illustrating an example of a configurationof the magnetic disk 11 according to the first embodiment. Note thatthis drawing illustrates an example of the rotation direction of themagnetic disk 11. The magnetic head 22 moves relative to the magneticdisk 11 by the rotation of the magnetic disk 11. Therefore, the writeand read direction, that is, the direction in which data is written orread by the magnetic head 22 along the circumferential direction isopposite to the rotation direction of the magnetic disk 11.

Servo information used for positioning of the magnetic head 22 iswritten to the magnetic disk 11 by, for example, a servo writer orself-servo write (SSW) in the manufacturing process. According to FIG. 2, as an example of the arrangement of servo areas in which the servoinformation is written, a plurality of servo areas SV arranged radiallyand at predetermined intervals in the circumferential direction isdrawn. A space between two servo areas SV consecutive in thecircumferential direction is used as a data area DA in which data iswritten.

Note that data written in the data area DA includes user data receivedfrom the host 2, metadata (for example, error correction code)accompanying the user data, system data, and others.

The plurality of servo areas SV include a plurality of normal servoareas NSV and a plurality of short servo areas SSV. According to theexample illustrated in FIG. 2 , the normal servo areas NSV and the shortservo areas SSV are alternately arranged in the circumferentialdirection. In other words, at least one or more short servo areas SSVare arranged between two normal servo areas NSV arranged consecutivelyin the circumferential direction, in other words, between two adjacentnormal servo areas NSV.

A plurality of concentric tracks 41 is set in the radial direction ofthe magnetic disk 11. In a data area DA, a plurality of data sectors iscontinuously formed along each track 41. The servo information writtenin the servo areas SV is used for a seek operation of moving themagnetic head 22 toward a target track 41 and a tracking operation ofmaintaining the magnetic head 22 on a target track 41.

Hereinafter, an area divided by a normal servo area NSV on a track 41 isreferred to as a normal servo sector NSV. An area divided by a shortservo area SSV on a track 41 is referred to as a short servo sector SSV.In addition, the normal servo sectors NSV and the short servo sectorsSSV are collectively referred to as servo sectors SV.

In addition, in a case where first data and second data are writtenalong the write and read direction and the first data is written in anarea to be read earlier than the second data, the direction as viewedfrom the area where the second data is written toward the area of thefirst data may be referred to as “preceding” or “ahead of” the areawhere the second data is written. Conversely, the direction as viewedfrom the area where the first data is written toward the area of thesecond data may be referred to as “subsequent” or “after” the area wherethe first data is written. In a case where it is described as “animmediately preceding second area” or “a second area disposedimmediately ahead of” when a first area is focused on, “the immediatelypreceding second area” or “the second area disposed immediately aheadof” refers to a second area through which the magnetic head 22 lastpasses before the magnetic head 22 passes through the first area. In acase where it is described as “an immediately subsequent second area” or“a second area disposed immediately after” when a first area is focusedon, “the immediately subsequent second area” or “the second areadisposed immediately after” refers to a second area through which themagnetic head 22 first passes after the magnetic head 22 passes throughthe first area.

In addition, in the circumferential direction, a front end of a certainarea may be referred to as the “start” of the area. Likewise, in thecircumferential direction, a rear end of a certain area may be referredto as an “end” of the area.

FIG. 3 is a diagram for describing an example of a positionalrelationship between the read head 22 r and the write head 22 waccording to the first embodiment. According to the example illustratedin this drawing, the read head 22 r and the write head 22 w are arrangedin a direction in which the actuator arm 15 extends. The read head 22 ris disposed closer to the rotation axis of the actuator arm 15 than thewrite head 22 w is. Then, in a state where the magnetic head 22 ispositioned on a certain track 41, the write head 22 w moves relative tothe magnetic disk 11 behind the read head 22 r.

That is, in the circumferential direction of the magnetic disk 11, thereis a gap between the read head 22 r and the write head 22 w, and thewrite head 22 w relatively moves with respect to the magnetic disk 11 insuch a manner as to be delayed from the read head 22 r by the gap. Thelength of the gap in the circumferential direction between the read head22 r and the write head 22 w is referred to as a read-write gap lengthRWgap.

Note that the read-write gap length RWgap varies depending on the skewangle of the magnetic head 22. Moreover, the skew angle of the magnetichead 22 varies depending on the radial position of the magnetic head 22.That is, the read-write gap length RWgap varies depending on the radialposition.

In the write operation of writing data to a data area of a target track,reading servo information from a servo area SV by the read head 22 r andwriting data to the data area DA by the write head 22 w are executedtemporally exclusively. However, since the write head 22 w moves in thecircumferential direction of the magnetic disk 11 behind the read head22 r by the read-write gap length RWgap, an area where data cannot bewritten may occur immediately before a servo sector SV in thecircumferential direction. Such an area where data cannot be written isherein referred to as an unwritable area UA.

In order to minimize the total capacity of the unwritable area UA in themagnetic disk 11, the controller 30 does not read the servo informationrecorded in a front part of a short servo sector SSV during the writeoperation.

That is, in the write operation, the short servo sectors SSV and thenormal servo sectors NSV are configured so that the length of a sectionin a short servo sector SSV where the servo information is read isshorter than the length of a section in a normal servo sector NSV wherethe servo information is read. Moreover, in the write operation, theshort servo sector SSV and the normal servo sector NSV are configured insuch a manner that the front part of the short servo sector SSV is notread.

Even when the read head 22 r reaches a short servo sector SSV in thewrite operation, the controller 30 can write data until the read head 22r reaches a rear part of the short servo sector SSV, and thus the lengthof the unwritable area UA present immediately before the short servosector SSV can be suppressed. As a result, the total capacity of theunwritable areas UA in the magnetic disk 11 can be suppressed.

Note that the servo information recorded in the front part of the shortservo sector SSV is read in the seek operation and the read operation.The seek operation is an operation of moving the magnetic head 22 in theradial direction toward a target track 41. The read operation is anoperation of reading data from a data area DA.

Although the lengths of the unwritable areas UA present immediatelybefore the short servo sectors SSV can be suppressed by theimplementation of the short servo sectors SSV, there are cases where theunwritable areas UA immediately before the short servo sectors SSVcannot be completely eliminated. In the first embodiment, with theconfiguration of the servo information recorded in the short servosectors SSV being devised, the lengths of the unwritable areas UApresent immediately before the short servo sectors SSV are furthersuppressed. Hereinafter, a specific example of the configuration of thenormal servo sectors NSV and the short servo sectors SSV will bedescribed.

FIG. 4 is a diagram illustrating an example of the configuration of anormal servo sector NSV according to the first embodiment. In thisexample, in the normal servo sector NSV, a preamble area PA2 in which apreamble #2 is written, a preamble area PA1 in which a preamble #1 iswritten, a servo mark area SM in which a servo mark is written, a Graycode area GC1 in which a Gray code is written, a pad area PAD in which aPAD is written, a burst area BN in which an N burst is written, a burstarea BQ in which a Q burst is written, and a post code area PC in whicha post code is written are arranged in the order mentioned in the writeand read direction.

The preamble #1 and the preamble #2 are information having a structureof a pattern of a single period that periodically changes in thecircumferential direction. The preamble #1 and the preamble #2 are usedto adjust the amplitude, the phase, and the frequency of sampling datawhen a servo waveform read by the read head 22 r is taken into the RWC25 as the sampling data on the basis of a servo clock.

During the read operation and the write operation, the preamble #2 isnot read, and only the preamble #1 is read. During the seek operation,in order to suppress deterioration in accuracy of servo waveformsampling data due to high-speed movement of the magnetic head 22 in theradial direction, the preamble #2 is also read in addition to thepreamble #1.

The servo mark is pattern data indicating the start of servoinformation. The controller 30 determines the timing to ingest varioustypes of servo information after the detection timing of the servo mark.

Note that, although details will be described later, a servo mark iswritten also in a short servo sector SSV. That is, a servo mark iswritten in each of the servo sectors SV regardless of whether the servosector SV is a normal servo sector NSV or a short servo sector SSV. Theintervals between the positions where the servo marks are written areconstant in the circumferential direction. Note that the intervalsbetween the positions where the servo marks are written may not beconstant in the circumferential direction.

A Gray code includes a cylinder address for identifying each track 41included in the magnetic disk 11 and a sector address for identifyingeach of the servo sectors SV on the track 41.

The PAD indicates a boundary between the Gray code area GC1 and theburst area BN.

The N burst and the Q burst are pattern data used to detect the amountof positional deviation of the center of the track 41 indicated by atrack number included in a Gray code #1. The N burst and the Q burst areexamples of burst data. The cylinder address included in the Gray codeis given as, for example, an integer value, and it is possible to obtainan offset amount of a decimal point based on the position indicated bythe cylinder address by demodulating the N burst and the Q burst. Thatis, the current position of the magnetic head 22 is obtained bydemodulating the N burst and the Q burst. The current position of themagnetic head 22 is the position of the magnetic head 22 on the magneticdisk 11 at the timing when the servo information is demodulated. Thecurrent position of the magnetic head 22 obtained by demodulating the Nburst and the Q burst is referred to as a demodulation position.

The post code indicates a correction amount of a positional deviation ofthe shape of the track 41 defined by the Gray code, the N burst, and theQ burst from the shape of an ideal track 41. The degree of thispositional deviation varies in synchronization with the rotation of themagnetic disk 11. Therefore, the positional deviation is also referredto as repeatable runout (RRO). That is, the post code is used to correctthe RRO.

The preamble area PA2, the preamble area PA1, the servo mark area SM,and the Gray code area GC1 of the servo sector SV are referred to as afront area FA in the sense of an area of a front part of the servosector SV. An area after the area FA in the servo sector SV is referredto as a rear area RA in the sense of an area after the servo sector SV.

That is, servo information (first information) including the preambles,the servo mark, and the Gray code is written in the front area. In therear area, servo information (second information) including the burstpatterns is written.

Note that the front area FA of the normal servo sector NSV is denoted asa front area FAn. A rear area RA of the normal servo sector NSV isdenoted as a rear area RAn.

In the write operation, the controller 30 reads the servo informationfrom an area after the preamble area PA1 in the normal servo sector NSV.It takes a predetermined period of time to switch the mode of themagnetic head 22 from a write mode, which is a mode in which writing ispossible, to a read mode, which is a mode in which reading is possible,and the magnetic head 22 continues to move in the circumferentialdirection during the switching. The distance that the magnetic head 22moves during the period of switching from the mode of writing data tothe mode of reading data is referred to as a write-read transitionlength (write-read transition length W2R in FIG. 4 ). That is, in a casewhere data is written to the data area DA and servo information is readfrom the servo area SV next time, a margin equal to or longer than atleast the write-read transition length W2R is required between theposition where the data writing is ended and the position where theservo information reading is started.

In addition, after the end of writing to the data area DA, degaussing ofcausing a write current, which attenuates at a certain frequency over apredetermined period of time, to flow through the write head 22 w isperformed. It becomes impossible to write significant data during thedegaussing. The length of an area where the degaussing is performed isreferred to as a degaussing length L_(DEG).

In FIG. 4 , writing of data to the data area DA is stopped at timingwhen the read head 22 r reaches a position P1 separated from the startposition P0 of the preamble area PA1 by the write-read transition lengthW2R in an opposite direction of the write and read direction. At timingwhen the writing is stopped, the write head 22 w is located at aposition P4 separated from the position P1 in the opposite direction ofthe write and read direction by the read-write gap length RWgap. Whenthe write head 22 w reaches the position P4, data writing is stopped,and degaussing is started. Then, the degaussing is executed until thewrite head 22 w reaches a position P3 separated from the position P4 inthe write and read direction by the degaussing length L_(DEG).

Therefore, in the example illustrated in FIG. 4 , it can be seen that anarea from the position P3 to a start position P2 of the preamble areaPA2 corresponds to an unwritable area UA.

FIG. 5 is a diagram illustrating an example of the configuration of ashort servo sector SSV according to the first embodiment. In thisexample, in the short servo sector SSV, a preamble area PA2 in which apreamble #2 is written, a preamble area PA1 in which a preamble #1 iswritten, a servo mark area SM in which a servo mark is written, a Graycode area GC2 and a Gray code area GC1 in which a Gray code is written,a pad area PAD in which a PAD is written, a burst area BN in which an Nburst is written, a burst area BQ in which a Q burst is written, and anadditional code area AC in which an additional code is written arearranged in the order mentioned in the write and read direction.

That is, the configuration of the short servo sector SSV is differentfrom the configuration of the normal servo sector NSV in that the Graycode area GC2 is added to a front area FA and that the additional codearea AC is included in the rear area RA instead of the post code areaPC. Note that the additional code area AC may not be included.

Note that a front area FA of the short servo sector SSV is denoted as afront area FAs. A rear area RA of the short servo sector SSV is denotedas a rear area RAs.

In the short servo sector SSV, the length of the front area FA is longerthan that of the normal servo sector NSV by the amount of the Gray codearea GC2 added to the front area FA.

In the write operation, the controller 30 reads the servo informationfrom the rear area RAs without reading the servo information in thefront area FAs. That is, the controller 30 starts reading the servoinformation from the timing when the read head 22 r reaches a startposition P10 of the rear area RAs. Therefore, the controller 30 stopswriting data to a data area DA at timing when the read head 22 r reachesa position P11 separated from the start position P10 of the rear areaRAs in the opposite direction of the write and read direction by thewrite-read transition length W2P.

At timing when the data writing is stopped, the write head 22 w islocated at a position P14 separated from the position P11 in theopposite direction of the write and read direction by the read-write gaplength RWgap. When the write head 22 w reaches the position P14, datawriting is stopped, and degaussing is started. Then, the degaussing isexecuted until the write head 22 w reaches a position P13 separated fromthe position P14 in the write and read direction by the degaussinglength L_(DEG).

That is, the distance (P13−P10) between the position P13 and the startposition P10 of the rear area RAs can be obtained according to thefollowing Equation (1).

P13−P10=W2R+RWGap−L _(DEG)   (1)

On the other hand, the distance (P12−P10) between the start position P12of the preamble area PA2 and the start position P10 of the rear area RAscan be obtained according to the following Equation (2). Note that, inthe following Equation (2), L_(PA2) represents the length of thepreamble area PA2, L_(PA1) represents the length of the preamble areaPA1, L_(SM) represents the length of the servo mark area SM, L_(GC2)represents the length of the Gray code area GC2, and L_(GC1) representsthe length of the Gray code area GC1.

P12−P10=L _(PA2) +L _(PA1) +L _(SM) +L _(GC2) +L _(GC1)   (2)

In the case illustrated in FIG. 5 , the distance (P13−P10) and thedistance (P12−P10) are equal. Therefore, there is no unwritable area UA.

Here, technology to be compared with the embodiment will be described.The technology compared with the embodiment is referred to as acomparative example. According to the comparative example, a front areaof a short servo sector has a similar configuration to that of the frontarea FAn of the normal servo sector NSV. That is, a front area accordingto the comparative example has a configuration in which the Gray codearea GC2 is omitted from the front area FAs illustrated in FIG. 5 . Insuch a case, the distance (P12′−P10) between a start position P12′ ofthe preamble area PA2 and a start position P10 of the rear area can beobtained according to the following Equation (3). However, the lengthsof the preamble areas PA2, the lengths of the preamble areas PA1, thelengths of the servo mark areas SM, and the lengths of the Gray codeareas GC1 are equal between the comparative example and the embodiment.

P12′−P10=L _(PA2) +L _(PA1) +L _(SM) +L _(GC2) +L _(GC1)   (3)

From Equations (2) and (3), it can be seen that the distance (P12′−P10)in the comparative example is shorter than the distance (P12−P10) in theembodiment by the length L_(GC2) of the Gray code area GC2. That is,according to the comparative example, there is a space between theposition P13 where the degaussing ends and the start position P12′ ofthe preamble area PA2, and this space is the unwritable area UAimmediately before the short servo sector SSV.

According to the embodiment, the length of the unwritable area UAimmediately before the short servo sector SSV can be reduced by thelength L_(GC2) of the Gray code area GC2 as compared with thecomparative example. As a result, the total capacity of unwritable areasUA in the magnetic disk 11 can be reduced.

Furthermore, according to the embodiment, as is clear from Equation (1),the position P13 at which the degaussing ends does not depend on thelength L_(GC2) of the Gray code area GC2. That is, the position P13 atwhich the degaussing ends is equivalent between the embodiment and thecomparative example. Therefore, according to the embodiment, the totalcapacity of the unwritable areas UA can be suppressed withoutdeteriorating the format efficiency.

Note that, in the example illustrated in FIG. 5 , the position P13 atwhich the degaussing ends coincides with the start position P12 of thepreamble area PA2. The position P13 at which the degaussing ends and thestart position P12 of the preamble area PA2 do not necessarily have tocoincide with each other, and an unwritable area UA having a slightlength in the circumferential direction may exist between the positionP13 at which the degaussing ends and the start position P12 of thepreamble area PA2.

For example, it is conceivable to make the length L_(GC2) of the Graycode area GC2 common to all the tracks 41 in the magnetic disk 11. Insuch a case, the designer may set the length L_(GC2) of the Gray codearea GC2 on the basis of the minimum value of the read-write gap lengthRWgap in the magnetic disk 11. Specifically, the designer may determinethe length L_(GC2) of the Gray code area GC2 in such a manner that thedistance (P13−P10) is equal to the distance (P12−P10) at a radialposition where the read-write gap length RWgap has the minimum value. Ina case where the length L_(GC2) of the Gray code area GC2 is determinedby this manner, at a radial position where the read-write gap lengthRWgap is not the minimum value, a space is generated between theposition P13 where the degaussing ends and the start position P12 of thepreamble area PA2, and this space is an unwritable area UA.

Alternatively, the designer may determine the length L_(GC2) of the Graycode area GC2 in such a manner that the distance (P13−P10) is slightlygreater than the distance (P12−P10) at a radial position where theread-write gap length RWgap has the minimum value. In a case where thelength L_(GC2) of the Gray code area GC2 is determined in this manner,an unwritable area UA is generated at any radial position, and thelength of the unwritable area UA is minimized at the radial positionwhere the read-write gap length RWgap has the minimum value.

Note that the designer may vary the lengths L_(GC2) of the Gray codeareas GC2 in the tracks 41 depending on the radial position so that thedistance (P13−P10) and the distance (P12−P10) are equal at any radialposition. In a case where the lengths L_(GC2) of the Gray code areas GC2in the tracks 41 are determined by such a manner, it is possible toeliminate the unwritable areas UA immediately before the short servosectors SSV at any radial position.

As described above, an additional code area AC is included in a reararea RAs of a short servo sector SSV.

In the write operation, the timing at which the N burst and the Q burstare demodulated from the rear area RAs of the short servo sector SSV isdetermined on the basis of an elapsed time with reference to the timingat which the servo mark is detected in an immediately preceding normalservo sector NSV instead of the servo mark written in the short servosector SSV itself. The elapsed time is counted by the counter 31, and anerror included in a count value increases as the count value of thecounter 31 increases. Therefore, in the short servo sector SSV, due tothe error included in the count value, the timing of demodulating the Nburst and the Q burst is shifted from originally intended timing, whichmay cause an unusual detection in which the demodulation position isgreatly shifted from an actual position. The additional code written inthe additional code area AC is data for correcting the demodulationposition obtained from the unusual detection.

FIG. 6 is a schematic diagram for describing a Gray code written in aGray code area GC1 of a normal servo sector NSV according to the firstembodiment.

In the example illustrated in FIG. 6 , a full Gray code includes a fullsector address on a higher order side and a full cylinder address on alower order side. The full sector address has a data structure includinga bit S0, a bit S1, and a bit S2 from the higher order side. The fullcylinder address has a data structure including a bit C0, a bit C1, abit C3, and a bit string Clow from the higher order side.

The full sector address S0 to S2 are divided into three, and the bitstrings C0 to C2 on the higher order side of the full cylinder addressare divided into three. Then, the bit S0, the bit C0, and the bit stringClow are written in the Gray code area GC1 of a heading normal servosector NSV among three normal servo sectors NSV arranged consecutivelyin the write and read direction from the higher order side. The bit S1,the bit C1, and the bit string Clow are written in the Gray code areaGC1 of a second normal servo sector NSV among the three normal servosectors NSV from the higher order side. The bit S2, the bit C2, and thebit string Clow are written in the Gray code area GC1 of a normal servosector NSV at the end among the three normal servo sectors NSV from thehigher order side.

That is, in the embodiment, a complete Gray code is distributed to threenormal servo sectors NSV. As a result, the length L_(GC1) of the Graycode area GC1 can be reduced, and the format efficiency can be improved.Instead, the controller 30 needs to demodulate Gray codes from the threeconsecutively-arranged normal servo sectors NSV in order to obtain thefull sector address and the full cylinder address. For example, at thetime of the seek operation, unless all the Gray codes can be demodulatedwithout errors from all the three consecutively-arranged normal servosectors NSV, the full cylinder address cannot be determined.

FIG. 7 is a schematic diagram for describing Gray codes written in Graycode areas GC1 and GC2 of short servo sectors SSV according to the firstembodiment.

In the example illustrated in FIG. 7 , the full sector address S0 to S2are divided into three, and the full cylinder address is not divided.Then, in a heading short servo sector SSV among the three short servosectors SSV consecutively arranged in the write and read direction, thebit S0 and the bit C0 are written in a Gray code area GC2 from thehigher order side, and the bit C1, the bit C2, and the bit string Cloware written in a Gray code area GC1 from the higher order side. In asecond short servo sector SSV among the three short servo sectors SSV,the bit S1 and the bit C0 are written in a Gray code area GC2 from thehigher order side, and the bit C1, the bit C2, and the bit string Cloware written in the Gray code area GC1 from the higher order side. In ashort servo sector SSV in the end among the three short servo sectorsSSV, the bit S2 and the bit C0 are written in a Gray code area GC2 fromthe higher order side, and the bit C1, the bit C2, and the bit stringClow are written in a Gray code area GC1 from the higher order side.

Since the Gray codes are written as described above, the controller 30can obtain the full cylinder address from any of the short servo sectorsSSV. If the Gray codes can be demodulated from one short servo sectorSSV without errors, the full cylinder address is determined, and thusthe frequency of occurrence of seek errors can be reduced.

In addition, at the time of the read operation, the controller 30 canpromptly complete the determination as to whether or not the fullcylinder address has been determined, and thus the read performance canbe improved.

Note that the writing methods of the Gray codes described in FIGS. 6 and7 are examples. In a case where the length L_(GC2) of the Gray code areaGC2 can be set to be further longer, the full Gray code may be writtenin the Gray code areas GC1 and GC2 of each of the short servo sectorsSSV. In such a case, since the controller 30 can obtain the full Graycode from each of the short servo sectors SSV in the read operation, thespeed required for positioning in the read operation is shortened.

That is, in the first embodiment, the amount of information of the Graycode obtained from a short servo sector SSV is greater than the amountof information of the Gray code obtained from a normal servo sector NSV.Furthermore, according to the first embodiment, the amount ofinformation of the Gray code obtained from a short servo sector SSV isgreater than that of the comparative example in which the Gray code areaGC2 is not included in the front area. Therefore, the read performanceis improved as compared with the comparative example. That is, amagnetic disk device with high performance can be obtained. In addition,the frequency of occurrence of seek errors is reduced as compared withthe comparative example, and a magnetic disk device with highperformance can be obtained.

The controller 30 generates a write gate that is an internal signalindicating a period during which writing to a data area DA is performed,a read gate that is an internal signal indicating a period during whichreading from the data area DA is performed, and a servo gate that is aninternal signal indicating timing of reading the servo information andoperates on the basis of these generated internal signals.

More specifically, when the magnetic head 22 passes through a certainservo sector SV, the processor 26 notifies the HDC 23 and the RWC 25 ofthe type of the servo sector SV and the type of the operation beingexecuted. The HDC 23 controls the write gate, the read gate, and theservo gate on the basis of the type of the servo sector SV notified fromthe processor 26 and the type of the operation being executed. The RWC25 causes the head IC 24 to execute reading and writing of user data anddemodulation of the servo information on the basis of the write gate,the read gate, the servo gate, and the notification from the processor26.

The controller 30 selectively uses five patterns of servo gates on thebasis of the type of the servo sector SV and the type of the operationbeing executed.

FIG. 8 is a schematic table for describing a relationship among the typeof the servo sector SV, the type of the operation being executed, and apattern of the servo gate to be used according to the first embodiment.

When the magnetic head 22 passes through a normal servo sector NSV inthe write operation and the read operation, a servo gate having awaveform of a pattern NormalSG1 is used. When the magnetic head 22passes through a normal servo sector NSV in the seek operation, a servogate having a pattern SeekSG1 is used.

When the magnetic head 22 passes through a short servo sector SSV in thewrite operation, a servo gate of a pattern ShortSG is used. When themagnetic head 22 passes through a short servo sector SSV in the readoperation, a servo gate of a pattern NormalSG2 is used. When themagnetic head 22 passes through a short servo sector SSV in the seekoperation, a servo gate having a pattern SeekSG2 is used.

FIG. 9 is a schematic diagram for describing an example of a waveform ofa servo gate of the NormalSG1 pattern used when the magnetic head 22passes through a normal servo sector NSV in the write operation of thefirst embodiment. In FIG. 9 , the temporal transition of the position ofthe read head 22 r in the circumferential direction is drawn. Therelationship among the position of the read head 22 r, the state of thewrite gate, and the state of the servo gate is further drawn.

When the read head 22 r reaches a position separated from the startposition of the preamble area PA1 in the opposite direction of the writeand read direction by the write-read transition length W2R (time t0),the HDC 23 prohibits writing of data by deasserting the write gate.

When the read head 22 r reaches the start position of the preamble areaPA1 (time t1), the HDC 23 asserts the servo gate.

The RWC 25 demodulates the servo information in a period in which theservo gate is asserted. When detecting the servo mark in the period inwhich the servo gate is asserted (time t2), the RWC 25 interprets asignal read by the read head 22 r and supplied from the head IC 24 in aperiod T1 from time t2 to time t3 as a Gray code and demodulates thesignal. The period T1 is a known period of time required for themagnetic head 22 to pass through the Gray code area GC1. The period T1is stored in advance in a desired storage device inside the RWC 25 oroutside the RWC 25 in association with the normal servo sector NSV. Whenthe processor 26 notifies that the type of the servo sector SV to passthrough is the normal servo sector NSV, the RWC 25 specifies the periodT1 that is a period associated with the normal servo sector NSV anddemodulates and acquires the signal acquired from the time t2 until theperiod T1 elapses as the Gray code.

After the time t3, when the read head 22 r reaches the end position ofthe post code area PC (time t4), the HDC 23 deasserts the servo gate.

FIG. 10 is a schematic diagram for describing an example of a waveformof a servo gate of the pattern ShortSG used when the magnetic head 22passes through a short servo sector SSV in the write operation of thefirst embodiment.

When the read head 22 r reaches a position separated from the startposition of the rear area RAs in the opposite direction of the write andread direction by the write-read transition length W2R (time t10), theHDC 23 prohibits writing of data by deasserting the write gate.

When the read head 22 r reaches the start position of the rear area RAs(time t11), the HDC 23 asserts the servo gate.

When the read head 22 r reaches the end position of the rear area RAs,that is, the end position of the additional code area AC (time t12), theHDC 23 deasserts the servo gate.

Since the servo gate of the pattern ShortSG is configured in thismanner, the controller 30 acquires no Gray codes when the magnetic head22 passes through the short servo sector SSV in the write operation.

FIG. 11 is a schematic diagram for describing an example of a waveformof a servo gate of the NormalSG1 pattern used when the magnetic head 22passes through a normal servo sector NSV in the read operation of thefirst embodiment. In FIG. 11 , the relationship between the position ofthe read head 22 r, the state of the read gate, and the state of theservo gate is drawn.

When the read head 22 r reaches immediately before the degaussing areaDEG (time t20), the HDC 23 prohibits reading of data by deasserting theread gate.

When the read head 22 r reaches the start position of the preamble areaPA1 (time t21), the HDC 23 asserts the servo gate.

When detecting the servo mark in the period in which the servo gate isasserted (time t22), the RWC 25 interprets a signal read by the readhead 22 r and supplied from the head IC 24 in a period T1 from time t22to time t23 as a Gray code and demodulates the signal, similarly to theoperation described using FIG. 9 .

When the read head 22 r reaches the end position of the post code areaPC (time t24), the HDC 23 deasserts the servo gate.

Therefore, when the magnetic head 22 passes through the normal servosector NSV in the read operation, the controller 30 acquires the Graycode from the Gray code area GC1 as in the case of the ride operation.

FIG. 12 is a schematic diagram for describing an example of a waveformof a servo gate of the pattern NormalSG2 used when the magnetic head 22passes through a short servo sector SSV in the read operation of thefirst embodiment.

When the read head 22 r reaches immediately before the degaussing areaDEG (time t30), the HDC 23 prohibits reading of data by deasserting theread gate.

When the read head 22 r reaches the start position of the preamble areaPA1 (time t31), the HDC 23 asserts the servo gate.

When detecting the servo mark in the period in which the servo gate isasserted (time t32), the RWC 25 interprets a signal read by the readhead 22 r and supplied from the head IC 24 in a period T2 from time t32to time t33 as a Gray code and demodulates the signal. The period T2 isa known period of time required for the magnetic head 22 to pass throughthe Gray code areas GC2 and GC1 and is longer than the period T1. Theperiod T2 is stored in advance in a desired storage device inside theRWC 25 or outside the RWC 25 in association with the short servo sectorSSV. When the processor 26 notifies that the type of the servo sector SVto pass through is the short servo sector SSV, the RWC 25 specifies theperiod T2, which is a period associated with the short servo sector SSV,and demodulates and acquires a signal acquired from the time t32 untilthe period T2 elapses as the Gray code.

After the time t33, when the read head 22 r reaches the end position ofthe rear area RAs, that is, the end position of the additional code areaAC (time t34), the HDC 23 deasserts the servo gate.

FIG. 13 is a schematic diagram for explaining an example of the waveformof a servo gate of the pattern SeekSG1 used when the magnetic head 22passes through a normal servo sector NSV in the seek operation of thefirst embodiment. In FIG. 13 , the relationship between the position ofthe read head 22 r and the state of the servo gate is drawn.

When the read head 22 r reaches the start position of the preamble areaPA2 (time t40), the HDC 23 asserts the servo gate.

When detecting the servo mark in the period in which the servo gate isasserted (time t41), the RWC 25 interprets a signal read by the readhead 22 r and supplied from the head IC 24 in a period T1 from time t41to time t42 as a Gray code and demodulates the signal, similarly to theoperation described using FIG. 9 .

After the time t42, when the read head 22 r reaches the end position ofthe post code area PC (time t43), the HDC 23 deasserts the servo gate.

FIG. 14 is a schematic diagram for explaining an example of the waveformof a servo gate of the pattern SeekSG2 used when the magnetic head 22passes through a short servo sector SSV in the seek operation of thefirst embodiment.

When the read head 22 r reaches the start position of the preamble areaPA2 (time t50), the HDC 23 asserts the servo gate.

When detecting the servo mark in the period in which the servo gate isasserted (time t51), the RWC 25 interprets a signal read by the readhead 22 r and supplied from the head IC 24 in a period T2 from time t51to time t52 as a Gray code and demodulates the signal, similarly to theoperation described using FIG. 12 .

After the time t52, when the read head 22 r reaches the end position ofthe additional code area AC (time t53), the HDC 23 deasserts the servogate.

FIG. 15 is a flowchart illustrating an example of the operation ofdetermining a demodulation method of the servo information of each ofthe servo sectors SV by the controller 30 according to the firstembodiment. Note that a series of operations illustrated in FIG. 15 isexecuted every time the magnetic head 22 passes through each of theservo sectors SV.

First, the controller 30 (for example, the processor 26) determineswhether or not the magnetic head 22 is about to pass through a shortservo sector SSV (S101). If the magnetic head 22 is about to passthrough a normal servo sector NSV instead of a short servo sector SSV(S101: No), the controller 30 (for example, the processor 26) determineswhether or not the type of the operation being executed is the seekoperation (S102).

If the type of the operation being executed is the seek operation (S102:Yes), the controller 30 (for example, the HDC 23) generates a servo gateof the pattern SeekSG1 (S103). As a result, the RWC 25 acquires the Graycode from the Gray code area GC1 (S104). Then, the operation ends.

If the type of the operation being executed is not the seek operationbut the tracking operation, that is, the operation of maintaining theposition of the magnetic head 22 on a target track 41 for the writeoperation or the read operation (S102: No), the controller 30 (forexample, the HDC 23) generates a servo gate of the pattern of NormalSG1(S105). As a result, the RWC 25 acquires the Gray code from the Graycode area GC1 (S104). Then, the operation ends.

If the magnetic head 22 is about to pass through a short servo sectorSSV (S101: Yes), the controller 30 (for example, the processor 26)determines whether or not the type of the operation being executed isthe seek operation (S106).

If the type of the operation being executed is the seek operation (S106:Yes), the controller 30 (for example, the HDC 23) generates a servo gateof the pattern SeekSG2 (S107). As a result, the RWC 25 acquires Graycodes from the Gray code areas GC1 and GC2 (S108). Then, the operationends.

If the type of the operation being executed is not the seek operationbut the tracking operation (S106: No), the controller 30 (for example,the processor 26) determines whether or not the type of the operationbeing executed is the write operation (S109).

If the type of the operation being executed is not the write operationbut the read operation (S109: No), the controller 30 (for example, theHDC 23) generates a servo gate of the pattern NormalSG2 (S110). As aresult, the RWC 25 acquires Gray codes from the Gray code areas GC1 andGC2 (S108). Then, the operation ends.

If the type of the operation being executed is the write operation(S109: Yes), the controller 30 (for example, the HDC 23) generates aservo gate of the ShortSG pattern (S111). As a result, the RWC 25acquires no Gray codes (S112), and the operation ends.

As described above, according to the first embodiment, each of the servosectors SV includes a front area FA in which the first information,including the preambles, the servo mark, and the Gray code, is writtenand a rear area RA in which the second information including the burstpatterns is written, the rear area RA disposed after the front area FA.The circumferential length of the front area of each of the short servosectors SSV is longer than the circumferential length of the front areaof a normal servo sector NSV by the amount of the Gray code area GC2added to the front area of each of the short servo sectors SSV.

In the read operation, the controller 30 can acquire a Gray code havinga larger amount of information than that of the comparative example froma short servo sector SSV, and thus the read performance is improved.That is, the performance is improved.

In addition, in the seek operation, the controller 30 can acquire a Graycode having a larger amount of information than that of the comparativeexample from a short servo sector SSV, and thus the occurrence frequencyof seek errors is reduced. That is, the performance is improved.

Furthermore, according to the first embodiment, the length of the frontarea FAs included in a short servo sector SSV is set on the basis of theread-write gap length RWgap.

For example, the length L_(GC2) of the Gray code area GC2 may bedetermined in such a manner that the distance (P13−P10) and the distance(P12−P10) are equal at the radial position where the read-write gaplength RWgap has the minimum value, and the length of the front area FAsmay be determined on the basis of the length L_(GC2) of the Gray codearea GC2 and Equation (2). The length L_(GC2) of the Gray code area GC2determined in this manner may be shared in the magnetic disk 11.

Alternatively, the length L_(GC2) of the Gray code area GC2 may bedetermined in such a manner that the distance (P13−P10) is slightlylarger than the distance (P12−P10) are equal at the radial positionwhere the read-write gap length RWgap has the minimum value, and thelength of the front area FAs may be determined on the basis of thelength L_(GC2) of the Gray code area GC2 and Equation (2). The lengthL_(GC2) of the Gray code area GC2 determined in this manner may beshared in the magnetic disk 11.

Alternatively, the length L_(GC2) of the Gray code area GC2 in each ofthe tracks 41 may be determined in such a manner that the distance(P13−P10) and the distance (P12−P10) are equal at any radial position,and the length of the front area FAs in each of the tracks 41 may bedetermined on the basis of the length L_(GC2) of the Gray code area GC2and Equation (2).

Furthermore, according to the first embodiment, a Gray code written inthe front area FAs of a short servo sector SSV includes a full cylinderaddress, and a Gray code written in the front area FSn of a normal servosector NSV includes a part of a cylinder address.

Therefore, the controller 30 can acquire the full cylinder address whenthe magnetic head 22 passes through the short servo sector SSV in theread operation. In addition, the controller 30 can acquire the fullcylinder address when the magnetic head 22 passes through the shortservo sector SSV in the seek operation.

Note that, in the first embodiment, the amount of information of theGray code written in the front area FAs of a short servo sector SSV islarger than that in the comparative example. The type of servoinformation whose amount of information is larger than that of thecomparative example is not limited to the Gray code.

For example, the preambles may be written in a longer area in the frontarea FAs of a short servo sector SSV, the area longer than that in thecomparative example, and the length of the front area FAs of the shortservo sector SSV may be increased accordingly. By making the area, fromwhich the preambles are obtained, longer, the accuracy of servo waveformsampling data to be captured in the RWC 25 is improved. As a result, thepositioning accuracy is improved. That is, the performance is improved.

Alternatively, the servo mark may be written in a longer area in thefront area FAs of the short servo sector SSV, the area longer than thatin the comparative example, and the length of the front area FAs of theshort servo sector SSV may be increased accordingly. By making the areawhere the servo mark is obtained longer, the frequency of occurrence ofdetection errors of the servo mark is reduced. As a result, thepositioning accuracy is improved. That is, the performance is improved.

As described above, the type of servo information whose amount ofinformation is increased as compared to that of the comparative examplemay be any of the preambles, the servo mark, or the Gray code.Alternatively, an area, in which desirable information different fromany of the preambles, the servo mark, and the Gray code is written, maybe added to the front area FAs of a short servo sector SSV, and thelength of the front area FAs of the short servo sector SSV may bethereby increased.

Second Embodiment

In a second embodiment, a magnetic disk device can read servoinformation by a read head in parallel with writing of data whilewriting the data by using a write head. The function of reading servoinformation in parallel with writing of data is referred to as aservo-read-during-write function. According to theservo-read-during-write function, the noise caused in a signal from theread head by the writing of the data by the write head is removed,thereby allowing writing of the data and reading of the servoinformation to be performed simultaneously.

In the magnetic disk device having the servo-read-during-write function,the length of an unwritable area UA generated due to a read-write gaplength RWgap is suppressed. Therefore, in the second embodiment,basically, all servo sectors SV are normal servo sectors NSV. Note that,a magnetic disk may include a short servo sector SSV also in the secondembodiment.

Hereinafter, the magnetic disk device of the second embodiment isdenoted as a magnetic disk device la, the magnetic disk of the secondembodiment is denoted as a magnetic disk 11 a, and a controller of thesecond embodiment is denoted as a controller 30 a. Then, mattersdifferent from those of the first embodiment will be described, anddescription of the same matters as those of the first embodiment will beomitted or briefly described.

FIG. 16 is a schematic diagram illustrating an example of aconfiguration of the magnetic disk 11 a according to the secondembodiment. As illustrated in the drawing, the magnetic disk 11 a isdifferent from the magnetic disk 11 of the first embodiment in that allthe servo sectors SV are normal servo sectors NSV.

FIG. 17 is a diagram illustrating an example of the configuration of anormal servo sector NSV according to the second embodiment. In thisexample, in the normal servo sector NSV, a preamble area PA2 in which apreamble #2 is written, a preamble area PA1 in which a preamble #1 iswritten, a servo mark area SM in which a servo mark is written, a Graycode area GC3 and a Gray code area GC1 in which a Gray code is written,an area PAD in which a PAD is written, a burst area BN in which an Nburst is written, a burst area BQ in which a Q burst is written, and apost code area PC in which a post code is written are arranged in theorder mentioned in the write and read direction.

As described above, in the magnetic disk device la having theservo-read-during-write function, the length of the unwritable area UAgenerated immediately before the servo sector SV due to the read-writegap length RWgap is suppressed. However, there are cases where theunwritable area UA cannot be completely eliminated immediately beforethe servo sector SV.

In the second embodiment, the length of the area in which the Gray codeis written is increased from the length of the Gray code area GC1 to thelength of the Gray code areas GC1 and GC3. As the length of the area inwhich the Gray code is written is increased, the length of theunwritable area UA is further suppressed.

Note that, in the example illustrated in FIG. 17 , there is nounwritable area UA immediately before the normal servo sector NSV. It isacceptable that an unwritable area UA of a slight length remainsimmediately before the normal servo sector NSV.

FIG. 18 is a schematic diagram for describing the Gray code written inthe Gray code areas GC1 and GC3 according to the second embodiment.

In the example illustrated in FIG. 18 , a Gray code having a structuresimilar to that of a normal servo sector NSV of the first embodiment iswritten in the Gray code area GC1.

Therefore, in a case of focusing only on the Gray code area GC1, thefull Gray code is distributed to three normal servo sectors NSV. Thecontroller 30 needs to demodulate the Gray code from the threeconsecutively-arranged normal servo sectors NSV in order to obtain thefull sector address and the full cylinder address.

In the Gray code area GC3, lower two bits of the full cylinder address,that is, a bit C1 and a bit C2 are written.

Therefore, it is possible to obtain the full cylinder address from oneof the three consecutively-arranged normal servo sectors NSV. Inaddition, it is possible to obtain a portion of the full cylinderaddress excluding the most upper bit from each of the other two of thethree normal servo sectors NSV.

That is, in the second embodiment, since the Gray code area GC3 isincluded in the normal servo sector NSV in addition to the Gray codearea GC1, the amount of information of the Gray code obtained from thenormal servo sectors NSV is increased.

In the controller 30 a, when the magnetic head 22 passes through acertain servo sector SV (that is, a normal servo sector NSV), theprocessor 26 notifies an HDC 23 and an RWC 25 of the type of theoperation being executed. The HDC 23 generates one of two patterns ofservo gates on the basis of the notified type of the operation beingexecuted. The RWC 25 controls a write gate, a read gate, and the servogate on the basis of the servo gate and the notified type of theoperation being executed.

FIG. 19 is a schematic table for describing a relationship between thetype of an operation being executed and a pattern of a servo gate to beused according to the second embodiment;

When the magnetic head 22 passes through a normal servo sector NSV inthe write operation and the read operation, a servo gate having awaveform of a pattern NormalSG3 is used. When the magnetic head 22passes through a normal servo sector NSV in the seek operation, a servogate having a pattern SeekSG3 is used.

FIG. 20 is a schematic diagram for describing an example of a waveformof a servo gate of the NormalSG3 pattern used when the magnetic head 22passes through a normal servo sector NSV in the write operation of thesecond embodiment. In FIG. 20 , the temporal transition of the positionof the read head 22 r in the circumferential direction is drawn. Therelationship among the position of the read head 22 r, the state of thewrite gate, and the state of the servo gate is further drawn.

When the read head 22 r reaches the start position of the preamble areaPA1 (time t60), the HDC 23 asserts the servo gate. As a result, the RWC25 starts demodulation of the servo information.

Note that, at time t60, the write gate is maintained in an assertedstate. Therefore, the RWC 25 starts demodulation of the servoinformation using the read head 22 r while continuing writing of datausing the write head 22 w.

When detecting the servo mark in the period in which the servo gate isasserted (time t61), the RWC 25 specifies the timing at which the readhead 22 r reaches the start position of the Gray code area GC1 (timet63) on the basis of elapsed time from the time t61. Then, the RWC 25interprets a signal read by the read head 22 r and supplied from a headIC 24 in a period T1 from the time t63 to time t64 as a Gray code anddemodulates the signal. The period T1 is a known period of time requiredfor the magnetic head 22 to pass through the Gray code area GC1. Theperiod T1 is stored in advance in a desired storage device inside theRWC 25 or outside the RWC 25 in association with the write operation. Ina case where it is notified that the type of the operation beingexecuted is the write operation, the RWC 25 specifies the period T1which is a period associated with the write operation and the time t63and regards the period T1 from the time t63 as a period for acquiringthe Gray code.

When the read head 22 r reaches a position separated from the startposition of the Gray code area GC1 in the opposite direction of thewrite and read direction by the write-read transition length W2R (timet62), the HDC 23 prohibits writing of data by deasserting the writegate.

As described above, according to the servo-read-during-write function,the noise caused in a signal from the read head 22 r by the writing ofthe data by the write head 22 w is removed, thereby allowing writing ofthe data and reading of the servo information to be performedsimultaneously.

Since the preamble #1, the preamble #2, and the servo mark have astructure that does not depend on the position in the radial direction,it is relatively easy to remove noise from the read preamble #1,preamble #2, and servo mark. However, the Gray code written in the Graycode area GC1 includes the cylinder address as described using FIG. 18 .That is, the Gray code written in the Gray code area GC1 has a structurethat depends on the position in the radial direction. Therefore, it isrelatively difficult to remove noise from the Gray code read from theGray code area GC1.

The use of the write head 22 w is halted until the read head 22 rreaches the Gray code area GC1 so that the Gray code can be acquiredfrom the Gray code area GC1 without errors as much as possible.Therefore, the write gate is deasserted at a position where the readhead 22 r goes back by the write-read transition length W2R from thestart position of the Gray code area GC1.

After the time t64, when the read head 22 r reaches the end position ofthe post code area PC (time t65), the HDC 23 deasserts the servo gate.

FIG. 21 is a schematic diagram for describing an example of a waveformof a servo gate of the NormalSG3 pattern used when the magnetic head 22passes through a normal servo sector NSV in the read operation of thesecond embodiment. In FIG. 21 , the relationship between the position ofthe read head 22 r, the state of the read gate, and the state of theservo gate is drawn.

When the read head 22 r reaches immediately before the degaussing areaDEG (time t70), the HDC 23 prohibits reading of data by deasserting theread gate.

When the read head 22 r reaches the start position of the preamble areaPA1 (time t71), the HDC 23 asserts the servo gate.

When detecting the servo mark in the period in which the servo gate isasserted (time t72), the RWC 25 interprets a signal read by the readhead 22 r and supplied from the head IC 24 in a period T3 from the timet72 to time t73 as a Gray code and demodulates the signal. The period T3is a known period of time required for the magnetic head 22 to passthrough the Gray code areas GC1 and GC3. The period T3 is longer thanthe period T1. The period T3 is stored in advance in a desired storagedevice inside the RWC 25 or outside the RWC 25 in association with boththe read operation and the seek operation. In a case where it isnotified that the type of the operation being executed is the readoperation, the RWC 25 specifies the period T3 which is a periodassociated with the read operation and regards the period T3 from thetime t73 as a period for acquiring the Gray code.

After the time t73, when the read head 22 r reaches the end position ofthe post code area PC (time t74), the HDC 23 deasserts the servo gate.

Therefore, when the magnetic head 22 passes through the normal servosector NSV in the read operation, the controller 30 a acquires the Graycode from the Gray code areas GC1 and GC3.

FIG. 22 is a schematic diagram for explaining an example of the waveformof a servo gate of the pattern SeekSG3 used when the magnetic head 22passes through a normal servo sector NSV in the seek operation of thesecond embodiment. In FIG. 22 , the relationship between the position ofthe read head 22 r and the state of the servo gate is drawn.

When the read head 22 r reaches the start position of the preamble areaPA2 (time t80), the HDC 23 asserts the servo gate.

When detecting the servo mark in the period in which the servo gate isasserted (time t81), the RWC 25 interprets a signal read by the readhead 22 r and supplied from the head IC 24 in a period T3 from the timet81 to time t82 as a Gray code and demodulates the signal. That is, in acase where it is notified that the type of the operation being executedis the seek operation, the RWC 25 specifies the period T3 which is aperiod associated with the seek operation and regards the period T3 fromthe time t81 as a period for acquiring the Gray code.

After the time t82, when the read head 22 r reaches the end position ofthe post code area PC (time t83), the HDC 23 deasserts the servo gate.

Therefore, when the magnetic head 22 passes through the normal servosector NSV in the seek operation, the controller 30 a acquires the Graycode from the Gray code areas GC1 and GC3.

FIG. 23 is a flowchart illustrating an example of the operation ofdetermining a demodulation method of the servo information of each ofthe servo sectors SV by the controller 30 a according to the secondembodiment. Note that a series of operations illustrated in FIG. 23 isexecuted every time the magnetic head 22 passes through each of theservo sectors SV.

First, the controller 30 a (for example, a processor 26) determineswhether the type of the operation being executed is the seek operation(S201).

If the type of the operation being executed is the seek operation (S201:Yes), the controller 30 a (for example, the HDC 23) generates a servogate of the pattern SeekSG3 (S202). As a result, the RWC 25 acquiresGray codes from the Gray code areas GC1 and GC3 (S203). Then, theoperation ends.

If the type of the operation being executed is not the seek operation(S201: No), the controller 30 a (for example, the processor 26)determines whether or not the type of the operation being executed isthe write operation (S204).

If the type of the operation being executed is not the write operationbut the read operation (S204: No), the controller 30 a (for example, theHDC 23) generates a servo gate of the pattern NormalSG3 for the readoperation (S205). As a result, the RWC 25 acquires Gray codes from theGray code areas GC1 and GC3 (S203). Then, the operation ends.

If the type of the operation being executed is the write operation(S204: Yes), the controller 30 a (for example, the HDC 23) generates aservo gate of the pattern NormalSG3 (S206). As a result, the RWC 25acquires the Gray code from the Gray code area GC1 (S207). Then, theoperation ends.

As described above, in the magnetic disk device la having theservo-read-during-write function, the Gray code area GC3 is included inaddition to the Gray code area GC1 in the normal servo sector NSV. As aresult, the amount of information of the Gray code obtained from thenormal servo sector NSV in the seek operation and the read operation isincreased.

For example, in a case where the Gray code is written in the Gray codeareas GC1 and GC3 by the method illustrated in FIG. 18 , when themagnetic head 22 passes through a specific servo sector SV, it ispossible to acquire the full cylinder address only by the magnetic head22 passing through the specific servo sector SV. Therefore, it ispossible to improve the read performance or to reduce the frequency ofoccurrence of a seek error. That is, the performance is improved.

Note that the type of servo information whose amount of information isincreased may be any of the preambles, the servo mark, or the Gray codeas in the first embodiment. Alternatively, an area in which desirableinformation different from any of the preambles, the servo mark, and theGray code is written may be added to the front area FA, and the lengthof the unwritable area UA may be suppressed.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

1. A magnetic disk device comprising: a magnetic disk comprising, aplurality of first servo sectors arranged in a circumferential directionat intervals, the plurality of first servo sectors each comprising, afirst area in which first information including a preamble, a servomark, and a Gray code is written, and a second area in which secondinformation including a burst pattern is written, the second areadisposed after the first area in a write and read direction along thecircumferential direction, and a plurality of data areas each disposedbetween two first servo sectors adjacent to each other in thecircumferential direction, wherein the plurality of first servo sectorscomprises a plurality of second servo sectors and a plurality of thirdservo sectors, one or more of the plurality of third servo sectors arearranged between two adjacent second servo sectors of the plurality ofsecond servo sectors, and a length of the first area comprised in eachof the plurality of third servo sectors in the circumferential directionis longer than a length of the first area comprised in each of theplurality of second servo sectors in the circumferential direction; amagnetic head; and a controller that, in a write operation of writingdata in one or more of the plurality of data areas using the magnetichead, demodulates the first information and the second information whenthe magnetic head passes through one of the plurality of second servosectors and demodulates the second information without demodulating thefirst information when the magnetic head passes through one of theplurality of third servo sectors and, in a read operation of readingdata from one or more of the plurality of data areas using the magnetichead, demodulates the first information and the second information whenthe magnetic head passes through one of the plurality of second servosectors and demodulates the first information and the second informationwhen the magnetic head passes through one of the plurality of thirdservo sectors.
 2. The magnetic disk device according to claim 1, whereinthe controller demodulates the first information and the secondinformation when the magnetic head passes through one of the pluralityof second servo sectors and demodulates the first information and thesecond information when the magnetic head passes through one of theplurality of third servo sectors in a seek operation of moving themagnetic head in a radial direction of the magnetic disk.
 3. Themagnetic disk device according to claim 1, wherein an amount ofinformation of the Gray code written in the first area comprised in eachof the plurality of third servo sectors is larger than an amount ofinformation of the Gray code written in the first area comprised in eachof the plurality of second servo sectors.
 4. The magnetic disk deviceaccording to claim 2, wherein an amount of information of the Gray codewritten in the first area comprised in each of the plurality of thirdservo sectors is larger than an amount of information of the Gray codewritten in the first area comprised in each of the plurality of secondservo sectors.
 5. The magnetic disk device according to claim 1, whereinthe magnetic head comprises a read head and a write head, the write headrelatively moves in the circumferential direction with respect to themagnetic disk with a delay of a first gap from the read head, and thecircumferential length of the first area comprised in each of theplurality of third servo sectors is set on a basis of the first gap. 6.The magnetic disk device according to claim 2, wherein the magnetic headcomprises a read head and a write head, the write head relatively movesin the circumferential direction with respect to the magnetic disk witha delay of a first gap from the read head, and the circumferentiallength of the first area comprised in each of the plurality of thirdservo sectors is set on a basis of the first gap.
 7. The magnetic diskdevice according to claim 1, wherein the magnetic head comprises a readhead and a write head, the write head relatively moves in thecircumferential direction with respect to the magnetic disk with a delayof a first gap from the read head, the first gap varies depending on aradial position of the magnetic disk, and the circumferential length ofthe first area comprised in each of the plurality of third servo sectorsis set on a basis of a minimum value of the first gap.
 8. The magneticdisk device according to claim 2, wherein the magnetic head comprises aread head and a write head, the write head relatively moves in thecircumferential direction with respect to the magnetic disk with a delayof a first gap from the read head, the first gap varies depending on aradial position of the magnetic disk, and the circumferential length ofthe first area comprised in each of the plurality of third servo sectorsis set on a basis of a minimum value of the first gap.
 9. The magneticdisk device according to claim 1, wherein the Gray code written in thefirst area comprised in each of the plurality of third servo sectorsincludes a full cylinder address, and the Gray code written in the firstarea comprised in each of the plurality of second servo sectors includesa part of the cylinder address.
 10. The magnetic disk device accordingto claim 2, wherein the Gray code written in the first area comprised ineach of the plurality of third servo sectors includes a full cylinderaddress, and the Gray code written in the first area comprised in eachof the plurality of second servo sectors includes a part of the cylinderaddress.
 11. A method for controlling a magnetic disk device comprisinga magnetic disk and a magnetic head, the magnetic disk comprising: aplurality of first servo sectors arranged in a circumferential directionat intervals, the plurality of first servo sectors each comprising: afirst area in which first information including a preamble, a servomark, and a Gray code is written; and a second area in which secondinformation including a burst pattern is written, the second areadisposed after the first area in a write and read direction along thecircumferential direction; and a plurality of data areas each disposedbetween two first servo sectors adjacent to each other in thecircumferential direction, wherein the plurality of first servo sectorscomprises a plurality of second servo sectors and a plurality of thirdservo sectors, one or more of the plurality of third servo sectors arearranged between two adjacent second servo sectors of the plurality ofsecond servo sectors, and a length of the first area comprised in eachof the plurality of third servo sectors in the circumferential directionis longer than a length of the first area comprised in each of theplurality of second servo sectors in the circumferential direction, themethod comprising: in a write operation of writing data in one or moreof the plurality of data areas using the magnetic head, demodulating thefirst information and the second information when the magnetic headpasses through one of the plurality of second servo sectors anddemodulating the second information without demodulating the firstinformation when the magnetic head passes through one of the pluralityof third servo sectors; and in a read operation of reading data from oneor more of the plurality of data areas using the magnetic head,demodulating the first information and the second information when themagnetic head passes through one of the plurality of second servosectors and demodulating the first information and the secondinformation when the magnetic head passes through one of the pluralityof third servo sectors.
 12. The method according to claim 11, furthercomprising: demodulating the first information and the secondinformation when the magnetic head passes through one of the pluralityof second servo sectors and demodulating the first information and thesecond information when the magnetic head passes through one of theplurality of third servo sectors in a seek operation of moving themagnetic head in a radial direction of the magnetic disk.
 13. The methodaccording to claim 11, wherein an amount of information of the Gray codewritten in the first area comprised in each of the plurality of thirdservo sectors is larger than an amount of information of the Gray codewritten in the first area comprised in each of the plurality of secondservo sectors.
 14. The method according to claim 12, wherein an amountof information of the Gray code written in the first area comprised ineach of the plurality of third servo sectors is larger than an amount ofinformation of the Gray code written in the first area comprised in eachof the plurality of second servo sectors.
 15. The method according toclaim 11, wherein the magnetic head comprises a read head and a writehead, the write head relatively moves in the circumferential directionwith respect to the magnetic disk with a delay of a first gap from theread head, and the circumferential length of the first area comprised ineach of the plurality of third servo sectors is set on a basis of thefirst gap.
 16. The method according to claim 12, wherein the magnetichead comprises a read head and a write head, the write head relativelymoves in the circumferential direction with respect to the magnetic diskwith a delay of a first gap from the read head, and the circumferentiallength of the first area comprised in each of the plurality of thirdservo sectors is set on a basis of the first gap.
 17. The methodaccording to claim 11, wherein the magnetic head comprises a read headand a write head, the write head relatively moves in the circumferentialdirection with respect to the magnetic disk with a delay of a first gapfrom the read head, the first gap varies depending on a radial positionof the magnetic disk, and the circumferential length of the first areacomprised in each of the plurality of third servo sectors is set on abasis of a minimum value of the first gap.
 18. The method according toclaim 12, wherein the magnetic head comprises a read head and a writehead, the write head relatively moves in the circumferential directionwith respect to the magnetic disk with a delay of a first gap from theread head, the first gap varies depending on a radial position of themagnetic disk, and the circumferential length of the first areacomprised in each of the plurality of third servo sectors is set on abasis of a minimum value of the first gap.
 19. The method according toclaim 11, wherein the Gray code written in the first area comprised ineach of the plurality of third servo sectors includes a full cylinderaddress, and the Gray code written in the first area comprised in eachof the plurality of second servo sectors includes a part of the cylinderaddress.
 20. The method according to claim 12, wherein the Gray codewritten in the first area comprised in each of the plurality of thirdservo sectors includes a full cylinder address, and the Gray codewritten in the first area comprised in each of the plurality of secondservo sectors includes a part of the cylinder address.